The invention relates to the preparation of terminal portions of optical fibers to be bonded, typically by soldering, into ferrules, connectors, laser diodes and related modules and devices. More particularly the present invention relates to the application of adherent metallic coatings to the ends of optical fibers to facilitate bonding of fiber ends at interfaces between the fiber ends and optoelectronic and related devices and modules including laser diodes. Adherent metallic coatings according to the present invention provide improved soldered joints having increased durability during repeated solder reflow to establish and maintain optimum alignment between light carrying optical fibers and optoelectronic packages and related devices to which they connect.
The use of optical fiber communication networks has grown to provide an alternative to coaxial cable systems. Optical fiber communication networks include optoelectronic modules for transmitting and receiving signals. In a typical arrangement, optical fibers direct optical signals to and from a suitably packaged optoelectronic device. A common structure includes an optical fiber solder sealed inside a nose tube that is brazed to the sidewall of the package. This type of hybrid electrical-optical package arrangement is commonly referred to as a xe2x80x9cfiber-pigtailedxe2x80x9d hybrid package. The process for interfacing the fiber to the package is called xe2x80x9cpigtailing.xe2x80x9d
An efficient optical fiber communications network requires proper alignment between an optical fiber and an optical subassembly. In an optoelectronic receiver, a fiber is aligned with an optical detector, typically a PIN photodiode. Light signal generation requires an optoelectronic transmitter using a light emitting diode (LED) or laser diode aligned with a suitable waveguide, such as an optical fiber. An optical fiber, correctly aligned, minimizes the amount of light attenuation within a subassembly.
The manufacture of an optoelectronic hybrid package requires precise alignment of a fiber optic member with an LED, a laser diode, or a photodetector. Alignment may thereafter be maintained using means to lock optical fibers inside optoelectronic packages. A variety of materials have been used to bond optical fibers to selected substrates including metal alloy solders. During their lifetime microelectronic solder joints show three key failure modes of overload failure, due to handling; thermal fatigue failure, during service; and dimensional changes, particularly for optoelectronic devices. The microstructure of the solder has an impact on each of the three failure modes after formation of the soldered bond. Changes in microstructure of a particular soldered bond may occur because of the composition of the solder, the chemical nature of the substrate and the manufacturing process used to form the solder joint (see Proceedings of Symposium for Process Design and Reliability of Solders and Solder Interconnections, Feb. 10-13, 1997, pages 49-58). Such changes may result in creep within a soldered joint connecting an optical fiber to an optoelectronic device.
Problems may occur with alignment and coupling efficiency to and from an optical fiber when there is movement due to creep in a joint used to lock an optical fiber inside an optoelectronic package. For this reason a need may exist for periodic adjustment of alignment between optical fibers and optically active devices. Realignment of optical fibers has been investigated in a variety of ways using soldered joints to hold optical fibers in required alignment with optical devices. United States Patent U.S. Pat. No. 4,119,363 describes hermetic sealing of an optical fiber inside a metal tube using solder. The solder, upon solidifying and cooling squeezes against the fiber and forms a hermetic seal. After passing the tube-fiber assembly through a hole in the wall of a device package and aligning the fiber with the device, a solder bond holds the tube to the wall to maintain optimum alignment within the device package.
United States Patents, U.S. Pat. No. 5,692,086 and U.S. Pat. No. 6,164,837 use commercially available gold sleeved optical fibers to be held in alignment with optical devices inside optoelectronic packages.
Optical fibers held in place by soldered joints may be realigned by solder reflow to soften the solder thereby allowing the optical fiber to be repositioned. In some cases, the reflow process introduces microstructural changes leading to embrittlement and eventually failure of the joint.
Soldered connections, in the form of optical fiber splices, terminations and hermetic seals, may include a thin metallic layer over the surface of an optical fiber adjacent to the position at which the splice, termination or seal will be made. Metal coating of terminal ends of optical fibers facilitates solder bonding and attachment of one optical fiber to another optical fiber, to a laser diode, to a ferrule and to connection points of optoelectronic devices.
United States Patent, U.S. Pat. No. 4,033,668 describes a method for joining a first glass member, such as an optical fiber, to a second member by means of solderable splices and terminations, which additionally can form hermetic seals. The splice, termination or seal may be formed after coating the peripheral surface of the glass member with a thin adhering metallic layer. After properly positioning the coated glass member, formation of a splice termination or seal with a corresponding member, may use heated solder to flow around the joint to form a bond between the members when cooled. When the second member is also formed of glass, a thin adhering metallic layer, similarly formed on the peripheral surface thereof, provides a solder receptive surface in the area of the intended joint. Metal may be applied to terminal portions of e.g. optical fibers by dipping them into a paste containing conductive metal particles.
United States Patent U.S. Pat. No. 5,100,507 addresses finishing techniques for lensed optical fibers. The process of finishing an optical fiber places an integral lens and a metallized outer coating on the end of an optical fiber. Metal may be deposited on the ends of optical fibers using known sputtering techniques. Materials deposited in this way include titanium, platinum and gold. Application of metal close to the lensed end of an optical fiber allows the formation of a soldered connection very close to the tip of the fiber. This limits subsequent movement of a lensed fiber relative to an aligned optical device.
Prior description of soldered connections involves individual processing of metallized ends of optical fibers. Optical fiber handling represents a challenge for the optical fiber industry. Manufacturing operations may include a number of steps requiring handling of long and short lengths of optical fiber. These lengths of optical fiber are fragile filaments requiring Careful handling and more efficient processes to accelerate the production of optical fibers, for communication links and related devices. With a growing demand for optical fiber systems and devices, there is a need for processing a plurality of optical fibers simultaneously.
The present invention provides a galvanic cell designed for precision application of metal to an array of nonconductors previously processed, by known electroless metal plating techniques, to produce a conductive layer on at least a portion of the nonconductor. The electrolytic plating equipment and process permit simultaneous electrolytic plating of a plurality of optical fiber tails, taking advantage of the conductive electroless metal fiber coatings as the cathodes of the electroplating cell. During operation of the electrolytic plating cell, pure metal such as nickel may be applied to increase the thickness of a previously deposited electroless metal layer thereby adding metal to e.g. an optical fiber that may subsequently be hermetically sealed in an optoelectronic package. Experimental refinement optionally accompanied by numerical modeling identified optimum design characteristics for the plating cell that includes a metal filament or bus bar, planar anodes, and a clamp fixture that stabilizes an array of optical fibers during plating. The resultant plating cell provides uniform deposition of metal down the axial length of each optical fiber.
In various optoelectronic packaging applications, where a hermetically sealed, soldered optical fiber feedthrough is required, there can be a need for the metallized fiber to survive multiple reflows of the solder to allow realignment of an optical fiber tip to an optoelectronic device. A common approach is to use gold/tin eutectic solder, which has a melting point of 280xc2x0 C. The metallized fiber may frequently be heated higher than this temperature. Conventional electroless nickel/immersion gold plating becomes embrittled near 300xc2x0 C. due to a phase transformation associated with presence of phosphorus in the electroless nickel deposit. The phase transformation causes the deposit to lose ductility. This leads to the initiation of cracking and failure of hermetically sealed, soldered joints that may impair the performance of a packaged optoelectronic device. Such device impairment may be overcome if the metallized fiber tip contains a minimum amount of electroless nickel. This is possible if the major constituent of the metal deposit, on the metallized optical fiber tip, is pure electroplated nickel. Pure nickel does not undergo the same phase transformation as electroless nickel.
An improved solder joint results from the use of electroless nickel as a very thin conductive layer over a stripped optical fiber. The conductive electroless metal deposit may be used as a cathode in an electroplating cell that adds a selected metal, e.g. pure nickel, over the electroless nickel. The presence of even small amounts of phosphorus, due to the use of electroless nickel, introduces the possibility of solder joint failure, as indicated previously. Further advantage could be gained by eliminating electroless nickel from metal coatings over nonconductors, such as optical fibers. This is possible using electroless silver as the conductive base layer. As such, the metal deposit survives repeated solder reflow cycles without substantial evidence of embrittlement. Preferably the use of silver provides a high yield of metallized optical fiber tips protected from embrittlement, as described herein. Other conductive base layers that are ductile and free from phase transformation may also be useful in applications consistent with the scope of the present invention
More particularly, the present invention provides an assembly for electroplating nonconductors having conducting portions. The assembly is an electrode assembly comprising an insulating frame having a first projection opposite a second projection. Preferably the insulating frame comprises a resin selected from the group consisting of acrylic resins, polyvinyl chloride resins and polycarbonate resins. A metal filament, for connection to the negative pole of a source of electrical energy, extends from the first projection to the second projection. Adjacent to the metal filament, a first conductive plate is attached to the insulating frame at a first distance from the metal filament. A second conductive plate attaches to the insulating frame at a second distance from the metal filament. The first plate and the second plate are adapted for connection to the positive pole of a source of electrical energy. The nonconductor is at least one optical fiber having a conducting portion in contact with the metal filament to provide connection of the conducting portion to a negative pole of the source of electrical energy. In one embodiment of an assembly according to the present invention, there is an array of a plurality of optical fibers each having a conducting portion in contact with the metal filament.
An assembly according to the present invention provides a conductive composite coating on the surface of a nonconductor. The composite coating comprises a first or base layer of a ductile metal in contact with the surface of the nonconductor. A second layer of metal is electroplated in contact with the base layer, and an outer metal layer overlies the second layer. The composite coating has durability sufficient to first form a soldered connection of the outer metal layer to a substrate and thereafter to survive at least 15 reflow cycles of solder of the soldered connection between a molten and a solid condition without formation of cracks or gaps in the composite coating. The composite coating has a base layer of an electroless metal, preferably electroless nickel or silver, a second layer of electroplated nickel and an outer metal layer of immersion gold. Preferably the nonconductor is an optical fiber.
The present invention also includes a method for forming a metallized portion on the surface of an optical fiber comprising a series of steps, beginning with providing an optical fiber having a glass portion free from protective buffer. Known processes are used for sensitizing the glass portion for electroless plating of metal thereon. A conducting portion of optical fiber results from electroless plating a first or base layer of a ductile metal in contact with the glass portion of the optical fiber. After connecting the conducting portion to the cathode of an electroplating cell, an electroplated second metal layer is plated in contact with the first layer to provide an electroplated portion of the optical fiber. Application of an outer metal layer overlying the second metal layer forms the metallized portion having durability sufficient for formation of a soldered connection of the outer metal layer to a substrate and thereafter to survive at least 15 reflow cycles of solder of the soldered connection between a molten and a solid condition without formation of cracks or bare spots in the metallized portion.
Definitions An xe2x80x9cassemblyxe2x80x9d according to the present invention may also be referred to herein as an xe2x80x9celectrode assemblyxe2x80x9d or a xe2x80x9cplating fixturexe2x80x9d that comprises an insulating frame accommodating a pair of conductive plates, as anodes, and a metal filament connected to the negative pole of a suitable source of electrical energy. The xe2x80x9cassemblyxe2x80x9d may include suitable wiring schemes for connection to a DC power supply.
The term xe2x80x9cmetal filamentxe2x80x9d or xe2x80x9cbus barxe2x80x9d refers to an electrically conductive component of an assembly for connection to the negative pole of a source of electrical energy, such as a power supply, to serve as a contact to other conductive structures that may be included in a cathode structure. Conductive structures include metallized nonconductors including optical fibers having metallized portions, particularly metallized optical fiber tips.
A xe2x80x9ccomposite coatingxe2x80x9d as used herein refers to at least two separate layers of metal applied successively over a nonconductor, such as an optical fiber. As applied to an optical fiber, a composite coating includes concentric layers of metal that may be applied using a common deposition technique or a combination of deposition techniques.
The term xe2x80x9coptical fiber corexe2x80x9d as used herein refers to the glass structure exposed by removal of buffer coating from a coated optical fiber.
Use of the terms xe2x80x9coverliesxe2x80x9d or xe2x80x9coverlying,xe2x80x9d when used to describe relationships among metal layers of a composite coating, means that a selected layer, e.g. an outer metal layer, has been formed over an underlying layer and may or may not be in contact with the underlying layer due to the possibility of intervening layers.
The term xe2x80x9celectroless metalxe2x80x9d refers to a layer of metal applied using known electroless plating techniques.
Use of the terms xe2x80x9celectroplated metalxe2x80x9d or xe2x80x9celectrolytic metalxe2x80x9d herein refers to metal layers applied using electroplating methods including that described herein using an assembly according to the present invention.
The term xe2x80x9cloadingxe2x80x9d refers to physical stress, in the form of tensile and compression forces and the like that may be applied to an optical fiber, particularly the metallized end of an optical fiber. Optical fiber loading produces defects such as cracks or gaps in a composite coating of metal layers formed over the end or tip of an optical fiber.
The term xe2x80x9cclampxe2x80x9d means a gripping device used to hold one or more nonconductors, especially optical fibers, in a prescribed location and orientation within an assembly according to the present invention. A clamp may include gripping faces having a covering of frictional, resilient material, to assist with fiber retention, or the gripping faces may include surface structure, such as grooves, or channels, to facilitate or maintain alignment of an array of optical fibers. After forming an organized array of optical fibers, a clamp may be used to maintain relative positioning between fibers.
A xe2x80x9csegmented anodexe2x80x9d is a multi-piece anode having at least two parts that, while electrically isolated from each other, may be connected to the same positive pole of a source of electrical energy.
The term xe2x80x9cadapted for connectionxe2x80x9d means that the cited elements, parts or structures include connectors, usually of a conventional type for connection to other parts or structures. In this case suitable adaptation allows connection to an electrical energy source, such as a battery or power supply.